Pointing to the rear or the side of the car, not forwards as the rushing incoming air may effect the burn, try and keep inlet and exhaust roughly the same length.
Silencers, exhaust one makes a small difference, intake makes a bigger difference.
Minimum burn time wants to be 30 mins to keep the burn chamber clean.
Fuel consumption is roughly 1/2 litre an hour at full power.
I have a cheap chinese one on my boat, works very well.